The present invention relates to a method and means for compensating for an abrupt pressure drop in a closed system for the circulation of a fluid, especially in the primary cooling circuit of a nuclear reactor plant. More particularly, the invention relates to improvements in safety systems which are used to automatically compensate for a pressure drop in closed circuits for the circulation of a fluid, such as in the primary cooling circuit of a nuclear reactor plant wherein the cooling medium is circulated by one or more internal or external centrifugal pumps.
It is well known to provide the primary cooling circuit in a nuclear reactor plant with a safety system which serves to compensate for an aburpt drop of system pressure. As a rule, the motor for the pump which circulates the fluid is arrested in immediate response to a predetermined drop of system pressure below the normal pressure. This is considered important and necessary since, when the drop of system pressure is attributable to a leak, further operation of the pump could result in uncontrolled escape of substantial quantities of contaminated fluid. The presently known safety systems embody means for automatically arresting the pump motor in response to a detected drop of fluid pressure. It has been found that such safety systems cannot prevent excessive leakage of fluid because the pressure drop often develops within a small fraction of a second whereas the rotary parts of a centrifugal pump often continue to rotate by inertia for a period of one or more minutes. Thus, the pump continues to deliver fluid which is free to escape from the closed circuit through one or more leaks. The danger which is inherent in such leakage will be readily appreciated when one considers that the capacity of a primary recirculating pump may reach or exceed 24,000 cubic meters per hour.
It is further known to provide closed circuits with rapidly closing valves which are capable of abruptly interrupting the flow of fluid in the circuit. Such valves are extremely complex and expensive and their operation must be controlled by complicated control systems whose complexity is another factor which is likely to contribute to extensive leakage in the event of malfunction.